1. Field of the Invention
The present invention relates to an image reading device in which an image recorded in a film, for example, is irradiated with a light beam and the image is read using a line sensor.
2. Description of the Related Art
Conventionally, in the image reading device, an exposure measurement is performed prior to a pre-scanning operation and a regular scanning operation, by which image data are sensed and output to a personal computer, so that an optimum exposure time, for which electric charges are accumulated in the line sensor, is calculated. Namely, in the exposure measurement, a scanning operation is carried out with a pitch, which is coarser than that of the regular scanning operation, and image data, sensed by exposing the line sensor with a relatively abort exposure time, is analyzed so that the optimum exposure time is obtained. The analysis of the image data is carried out based on a histogram representing a frequency distribution of pixel values of the image data obtained by the line sensor.
The histogram is stored in a memory. Namely, in the memory, the frequency of occurrence, i.e. the number of pixels having an identical pixel value (an identical luminance value, for example), is stored in an address corresponding to the pixel value. In the calculation of the optimum exposure time, however, as the entire range of data forming the histogram is not utilized, only the data included in a predetermined range are required.